1. Field of the Invention
The present invention relates to optical multiplexing/demultiplexing apparatus and method therefor for use in an optical communication network to which wavelength division multiplexing (WDM) techniques are applied, and more particularly, to optical multiplexing/demultiplexing apparatus and method therefor, in which an optical power balance of each wavelength light of a wavelength division multiplexed signal light can be retained at input and output ports of the optical multiplexing/demultiplexing apparatus.
2. Description of the Related Art
A WDM system which utilizes band characteristics of optical fibers is a transmission system that is suitable for constructing an optical network which is flexible in taking in and out signal lights. Such an optical network permits not only the transmission of a wavelength division multiplexed signal from a point to a point but also the selective transmission of only signal light of a certain wavelength in a wavelength division multiplexed signal light and the branching or insertion of signal light of the other wavelengths by an optical multiplexing/demultiplexing apparatus provided at some midpoint on a transmission path.
FIG. 10 illustrates an example of an optical communication system (namely, a WDM network system), which is networked by using conventional optical multiplexing/demultiplexing apparatuses.
In the conventional system of FIG. 10, at least a pair of optical fibers are used as transmission paths for upstream and downstream communication lines (or circuits). Further, a plurality of optical amplifier repeaters are placed for compensating losses in optical fibers. Each of the optical amplifier repeaters is provided with at least two optical amplifiers for the upstream and downstream communication lines. Moreover, a plurality of signal lights (namely, WDM signal lights) of different wavelengths are transmitted from each terminal device to a single optical fiber. Furthermore, the transmitted WDM signal lights are assigned transmission paths thereto according to the wavelength thereof, respectively, by the optical multiplexing/demultiplexing apparatuses, then, sent to a receiving terminal device.
The conventional optical multiplexing/demultiplexing apparatuses used in such a WDM network system are constituted by combining optical add-drop multiplexer (OADM) circuits as illustrated in FIG. 11.
In the OADM circuit of FIG. 11, among WDM signal lights propagating in a main optical fiber, signal lights of some wavelengths are branched to branch optical fibers. Further, signal lights of the remaining wavelengths are multiplexed with a signal light input from an insertion optical fiber, then to be output to the main optical fiber. This OADM circuit can be constituted by using wavelength multiplexing/demultiplexing elements, for instance, a dielectric multilayer filter, a WDM coupler, a fiber grating and an arrayed waveguide grating (AWG). Hitherto, there have been proposed ADM circuits of various constitutions (see, for example, Miyakawa et al., xe2x80x9cA Study of Optical ADM using Dielectric Filterxe2x80x9d, Denshi Joho Tsushin Gakkai Sogo Taikai, vol. B-10-234, 1997; xe2x80x9cOptical ADM and Wavelength Routing Based on WDM Systemxe2x80x9d, by the applicant of the instant application, Denshi Joho Tsushin Gakkai Sogo Taikai, vol. B-1158, 1996; and Mizuochi et al., xe2x80x9cA Study on Optical Add-Drop Multiplexers Using Fiber Grating Filtersxe2x80x9d, Denshi Joho Tsushin Gakkai Tsushin Society Taikai, vol. SB-9-5, 1995).
In an actual WDM network system, there is the necessity for using at least one optical fiber pair paths for the upstream and downstream lines. Therefore, the optical multiplexing/demultiplexing apparatus is constituted by at least two OADM circuits as illustrated in FIG. 11. FIG. 12 shows an example of the conventional optical multiplexing/demultiplexing apparatus constituted by two OADM circuits.
The aforementioned conventional optical multiplexing/demultiplexing apparatus, however, has encountered the problem that there is caused a difference in optical power level between signal light (hereunder sometimes referred to as through light) passing through the OADM circuit and signal light (hereunder sometimes referred to as add light) inserted in the OADM circuit.
In the case that the optical power level is balanced between through light and add light included in signal light passing through the OADM circuit as shown in FIG. 13(A), the WDM signal light is normally transmitted between the terminal devices. However, the balance between the optical power levels of the through light and the add light included in the signal light passing through the OADM circuit is lost when the power of the WDM signal light or the add light input to the OADM circuit decreases owing to an increase in loss in the optical fiber provided in a pre-stage of the optical multiplexing/demultiplexing apparatus or to lowering of an output of the optical amplifier repeater. For instance, in the case where the power level of the add light is extremely smaller than that of the through light as illustrated in FIG. 13(B), the add light is buried in noises caused by the optical amplifier repeater at a repeater section provided in a post-stage of the optical multiplexing/demultiplexing apparatus. Consequently, the transmission characteristics are extremely deteriorated.
As a system for reducing the difference in optical power level between through light and add light, there have been proposed systems for regulating the optical power of add light transmitted from a terminal device to an optical multiplexing/demultiplexing apparatus, by controlling an operation of an optical amplifier repeater placed between the terminal device, which transmits add light, and the optical multiplexing/demultiplexing apparatus. Practically, an example of such systems is such that an amplifying operation of an optical amplifier repeater is controlled by transmitting a supervisory control signal from a terminal device to an optical multiplexing/demultiplexing apparatus. Alternatively, another example of such systems is that a dummy light of different wavelengths is transmitted from the terminal device in addition to add light, and the optical power level of the add light output from the optical amplifier repeater is controlled by regulating a power level of the dummy light. These systems, however, have encountered the problems that the systems cannot preserve the balance between the optical power levels of the through light and the add light in the case that an abnormality, such as, increase in loss of optical fiber, occurs between a terminal device and an optical amplifier repeater or between an optical amplifier repeater and an optical multiplexing/demultiplexing apparatus, and that there is a limit to the range of the optical power of add light, which can be controlled by an optical amplifier repeater.
The present invention is accomplished in view of the aforementioned problems. Accordingly, an object of the present invention is to provide optical multiplexing/demultiplexing apparatus and method therefor in which an optical power of each wavelength signal light of a wavelength division multiplexed signal light can be retained at input and output ports by controlling a power level of an insertion light in the optical multiplexing/demultiplexing apparatus.
To achieve the foregoing object and in accordance with an aspect of the present invention, there is provided an optical multiplexing/demultiplexing apparatus comprising a branching/inserting device connected to a transmission path where a wavelength division multiplexed signal light is transmitted, and being capable of branching a signal light of at least one wavelength from a signal light transmitted on the transmission path, and being capable of inserting a signal light of at least one wavelength into a signal light transmitted on the transmission path, wherein the optical multiplexing/demultiplexing apparatus further comprises an insertion-light power control device for controlling an optical power of an insertion light based on an optical power of a branch light branched by the branching/inserting device, so that a ratio between the optical power of the branch light branched by the branching/inserting device, and the optical power of the insertion light inserted by the branching/inserting device is constant.
With such a constitution, the optical power of the insertion light is controlled by the insertion-light power control device based on the optical power of the branch light branched by the aforesaid branching/inserting device, so that a ratio between the optical power of the transmission light and the optical power of the branch light at an input port of the branching/inserting device is equal to a ratio between the optical power of the transmission light and the optical power of the insertion light at an output port of the branching/inserting device. Consequently, an optical power of each wavelength light of a wavelength division multiplexed signal light at each of input and output ports of the optical multiplexing/demultiplexing apparatus can be maintained at a constant level. Therefore, good transmission quality of wavelength division multiplexed signal light can be maintained.
Further, a practical embodiment of the abovedescribed optical multiplexing/demultiplexing apparatus may be constituted so that the branching/inserting device comprises: a signal light branching portion for sending a signal light of at least one wavelength included in a wavelength division multiplexed signal light travelling on the transmission path, to the insertion-light power control device as a branch light and for outputting a signal light of the other wavelengths as a transmission light; and a signal light inserting portion for multiplexing the transmission light from the signal light branching portion with the insertion light sent from the insertion-light power control device, to output a multiplexed light to the transmission path, and the insertion-light power control device comprises: a branch light power detecting portion for detecting the optical power of the branch light from the signal light branching portion; an insertion-light power detecting portion for detecting the optical power of the insertion light input from the exterior; an insertion-light power regulating portion for regulating the optical power of the insertion light; and a control portion for controlling an operation of the insertion-light power regulating portion, based on the optical power of the branch light detected by the branch light power detecting portion and on the optical power of the insertion light detected by the insertion-light power detecting portion.
Further, another practical embodiment of the optical multiplexing/demultiplexing apparatus may be constituted so that a plurality of the aforesaid branching/inserting devices are provided corresponding to a plurality of main transmission paths, respectively, and a plurality of the insertion-light power control devices are provided corresponding to the plurality of main transmission paths, respectively. Thus, for each of the main transmission paths, the relative optical power of each wavelength light of the wavelength division multiplexed signal light at the input and output ports of the optical multiplexing/demultiplexing apparatus can be maintained at a constant level.
Moreover, the aforesaid optical multiplexing/demultiplexing apparatus may further comprises: a branch light power control device for controlling the optical power of each of branch lights respectively having wavelengths different from one another branched by the branching/inserting device; a branch light multiplexing device for multiplexing branch lights output from the branch light power control device and for outputting the multiplexed light to one of a pair of branch transmission paths; and an insertion light demultiplexing device for demultiplexing a signal light which is obtained by multiplexing each insertion light inserted by the branching/inserting device and is sent from the other of the pair of branch transmission paths corresponding to wavelengths thereof and for outputting the demultiplexed light to the insertion-light power control device, so that the optical power of each branch light included in output signal light of the branch light multiplexing device is controlled to be constant.
With such a constitution, the branch light sent from each of the branching/inserting device is multiplexed at a constant power. Then, the multiplexed branch light is output to the branch transmission path. Moreover, the multiplexed insertion light inserted from the branch transmission path is demultiplexed for each wavelength, and then sent to the insertion-light power control device.
Accordingly, it is possible to cope with a pair of branch transmission paths, such as an optical fiber pair paths. Further, the relative optical power of each branch light is controlled to be constant. Thus, the optical power balance among respective wavelength signal lights of the wavelength division multiplexed signal light propagating the branch transmission paths can be retained constant.
Furthermore, as a practical constitution of the branch light power control device may comprise: an optical power detecting portion for detecting the optical power of each of the branch lights branched by the branching/inserting device; an optical power regulating portion for regulating the optical power of each of the branch lights; and an optical power control portion for controlling an operation of the optical power regulating portion based on the optical power of each of the branch lights detected by the aforesaid optical power detecting portion.
In addition, the constitution may be such that an operation of the optical power regulating portion is controlled by the optical power control portion based on the optical power of each of the branch lights detected by the optical power detecting portion of each insertion-light control device. Thus, the branch light power detecting portion of the insertion-light control device can be used in common as the optical power detecting portion of the branch light power control device. Consequently, the constitution of the branch light power control device is simplified.
Furthermore, in accordance with another aspect of the present invention, there is provided an optical multiplexing/demultiplexing method for enabling the branching of a signal light of at least one wavelength from a signal light travelling on a transmission path where a wavelength division multiplexed signal light is transmitted, and inserting of a signal light of at least one wavelength into a signal light travelling on the transmission path, wherein an optical power of an insertion light inserted into a signal light travelling on the transmission path is controlled based on an optical power of a branch light branched from the signal light travelling on the transmission path so that a ratio between the optical power of the branch light and the optical power of the insertion light is constant.
Other features, objects and advantages of the present invention will become apparent from the following description of preferred embodiments with reference to the accompanying.